Generally, ignition control and fuel injection control are performed in engine operation by determining cylinders on the basis of output signals from a crank angle sensor and a cam angle sensor and detecting a crank angle. However, a cylinder for initial ignition/injection is not known at the start of an engine until the engine is cranked by a starter and determination of a specified cylinder is completed, that is, a signal of a predetermined crank angle of the specified cylinder is detected.
In order to solve such a problem, as disclosed in patent document 1 (JP-A-60-240875), a starting quality and exhaust emission at the start are improved by storing a crank angle (a stop position of a crankshaft) at the time of engine rotation stop in a memory, and starting ignition control and fuel injection control on the basis of a crank angle at the time of engine rotation stop, which is stored in the memory, at a subsequent engine start until a signal of a predetermined crank angle of a specified cylinder is initially detected.
Since an engine is rotated by inertia for some time after an ignition switch is turned off (operated to OFF position) to stop ignition and fuel injection, a crank angle at an actual engine rotation stop (at a subsequent engine start) is erroneously determined in the case where a crank angle at the time of OFF-operation of an ignition switch is stored. Accordingly, it is necessary to maintain an electric source of a control system in an ON state to continue detection of a crank angle until engine rotation is completely stopped even after the ignition switch is turned off. However, a crank angle at the time of engine rotation stop cannot be exactly detected since a phenomenon, in which engine rotation is reversed by a compression pressure in a compression stroke, is generated just before engine rotation is stopped (reverse rotation cannot be detected).
Also, as disclosed in patent document 2 (JP-A-11-107823), an initial injection cylinder and an initial ignition cylinder at a subsequent engine start are determined by estimating a cylinder, into which fuel is injected just before an ignition switch is turned off, and an engine rotation stop position on the basis of an operating state at that time, and determining an initial position of a crankshaft at a subsequent engine start from the estimated stop position.
Engine rotation is stopped at a position (a position of torque=0), in which a negative torque in a compression stroke and a positive torque in an expansion stroke of other cylinders balance each other, at the time of engine rotation stop provided that no friction is present in an engine. However, engine friction is actually present to cause a stop position to vary in a relatively wide range of crank angle, in which torque is below engine friction. Therefore, with the technique of patent document 2, it is difficult to accurately estimate an engine rotation stop position, with the result that there is a possibility of erroneously determining an initial injection cylinder and an initial ignition cylinder at the time of engine starting. Thus, it is difficult to improve a starting operation and exhaust emission at the start.
Also, with patent document 2, an initial cylinder in successive injection at a subsequent engine start is estimated by calculating rotation (TDC number) until a crankshaft is rotated by inertia to be stopped, on the basis of an engine operating state (intake pipe pressure, engine rotational speed) at the moment when an ignition switch is turned off, and estimating an engine rotation stop position from a cylinder, into which fuel is injected just before an ignition switch is turned off, and rotation (TDC number) until the stoppage.
Since according to patent document 2, only kinetic energy of inertia of an engine is previously subjected to matching to be stored and variation in kinetic energy is not predicted in the course of stop, variation due to fabrication tolerance of engines, changes with the passage of time, and changes in engine friction (for example, a difference in viscosity due to temperature change of an engine oil) causes a possibility that rotation (TDC number) until a crankshaft is rotated by inertia to be stopped is erroneously estimated. Therefore, with patent document 2, it is difficult to accurately estimate an engine rotation stop position, with the result that an initial injection cylinder and an initial ignition cylinder at the time of engine starting are erroneously determined to worsen a starting quality and exhaust emission at the start.
Further, in order to perform control conforming to an operation condition in internal combustion engines, it is necessary to grasp a quantity of kinetic energy, which an internal combustion engine has. Conventionally, an engine rotational speed is widely used in engine control as a value representative of kinetic energy. According to, for example, patent document 2 (JP-A-11-107823), rotation (TDC number) until a crankshaft is rotated by inertia to be stopped is calculated on the basis of an engine operating state (intake pipe pressure, engine rotational speed) at the moment when an ignition switch is turned off, and an initial cylinder in successive injection at a subsequent engine start is estimated from a cylinder, into which fuel is injected just before the ignition switch is turned off, and rotation (TDC number) until the stoppage.
Also, according to patent document 3 (JP-A-2001-82204), it is determined during execution of fuel cut-off in deceleration whether an engine can be driven by an electric motor (motor/generator or the like) at a rotational speed higher by a predetermined speed ÄNe than a normal rotational speed Ne1 for a fuel supply return from the fuel cut-off. In the case where driving is possible, the fuel return rotational speed is set to a low rotational speed Ne2 to improve fuel consumption, and in the case where driving is not possible, the fuel return rotational speed is set to the normal fuel return rotational speed Ne1.
According to patent document 2, however, kinetic energy of inertia of an engine is previously subjected to hing to be stored and variation in kinetic energy is not predicted in the course of stop, in the same manner as in patent document 2. Accordingly, variation due to changes in engine friction (for example, a difference in viscosity due to temperature change of an engine oil) causes a possibility that rotation (TDC number) until a crankshaft is rotated by inertia to be stopped is erroneously estimated. Besides, in the case where deviation from a constant subjected to matching is generated due to changes with the passage of time, or the like, correction cannot be made.
Also, according to the disclosure of patent document 3, only a fuel supply return rotational speed is prepared as a determination condition of fuel return but variation in rotational speed, that is, variation in kinetic energy is not predicted. Accordingly, a fuel supply return rotational speed is set to a rather high level as means for avoiding engine stall. Thus, an effect of fuel consumption must be sacrificed.